In order to get the prickly heads with my usual dark background, I decided to paint the background first and then use Acrylic Gouache to paint parts of the flowers.
In order to identify the flower shapes, I painted a strong wash over the whole paper, using a mixture of Anthraquinachridone, and Ultramarine blues with some Pthalo Blue and Green. I also added some Indigo into the bottom dark corners. I realised that I would not be able to change the background once I had started painting the flowers, so it was important to get the strength right, remembering that it would dry paler in colour!
Whilst the wash was still very wet, I used Sea Salt to identify the flowers, sprinkling generous amounts in circles, where I wanted the flower heads to be. At this stage the salt looks very dark as it absorbs the pigment beneath it. I left the painting overnight, as it is vital that the salt is absolutely dry before it is removed.
The following morning I used a small plastic glue spatula to remove the salt from the paper.
I lightened each flower on the top and to the right and finished of with Acrylic Gouache to give the highlights
I added the leaves and the stems, and finally added a dark wash of Ultramarine Blue and Moonglow to the darker side of each globe for the shadows. Whilst this was still wet I scratched a little paint out from the wash to give some dark spikes to the undersides. I used the ferrule of an old paint brush which had lost all its hairs!
I am pleased with the outcome and really like the pale passage in the background in the middle of the lower half of the painting, but I wish I had had the courage to make the dark background passages even darker.